1. Field of the Invention
The present invention relates to a dry toner for developing static charge images formed by electrophotography. More particularly, it relates to a toner for low energy fixing which is required in the case of developing static charge images in a hot roll fixing system at a decreased hot roll temperature, a decreased roll pressure, an increased rotation number of rolls or the like conditions.
2. Description of Related Arts
Recently, various alterations or modifications on copiers or printers utilizing electrophotography have been desired in accordance with their popularization. For example, it has been desired to render them to work at low energy in order to allow them to find their way into almost all homes, to render the machines to run at high speeds with view to promoting the spread of the machines into so-called gray areas positioned at a borderline between printers and copiers, to decrease the roll pressure of the machines aiming at simplification of fixing rolls in order to reduce the cost of the machines, and further to reduce the energy required for running the machines to a low level accompanied by the development of multifunctional copiers and printers.
Of the above-described requirements, it is particularly desired for a toner for developing static charge images (hereafter, referred to simply as "toner") that copying machines or printers are run at low fixing energy. Conventionally, investigation for decreasing fixing energy has been focused on the resin contained in the toner and various toners have been proposed which include, for example, those whose molecular weight an/or molecular weight distribution are/is improved, those whose composition has been changed, those which contain one or more additives in the resin.
In order to change the molecular weight and/or molecular weight distribution of the toner resin, a method has been used in which the molecular weight distribution is broadened by decreasing the molecular weight of the lower molecular weight portions of the resin while increasing the molecular weight of the higher molecular weight portions of the resin. Also, a method has been used for this purpose in which the higher molecular weight portions of the resin is crosslinked. However, these methods cause problems that it is necessary to decrease the glass transition temperature of the resin to be used in order for the resulting toner to have a sufficient fixing property and as the result it is inevitable that the storage stability of the toner containing such a resin as described above is deteriorated. In addition, it is difficult to make a toner to have a sufficient fixing property even if efforts are made to improve the fixing property of the toner while maintaining the storage stability thereof.
Another method which has been proposed for lowering the energy upon fixing is a method in which an additional resin or sub-resin is added to the resin contained in the toner as the second component in order to increase the fixing property of the toner. Although this method, which includes the addition of a sub-resin which has a high crystallinity with view to maintaining the glass transition temperature of the resin composition, gives rise to good results with respect to the fixing property and storage stability of the toner, it causes other problems that the presence of a highly crystalline sub-resin results in an abrupt decrease of the melt viscosity of the toner upon hot roll fixing and that offset phenomenon tends to occur because the cohesive force of the toner upon melting is weak. In order to obviate the above problems, it has been proposed to add polyolefin wax which has a high crystallinity to the toner composition. This causes a further problem that components to be dispersed in the toner such as carbon and an charge control agent are difficult to be dispersed in the polyolefin wax, which makes it unsuccessful to obtain a toner having a uniform composition.